Compare your current and new tire size, check diameter and width changes, calculate speedometer error, estimate clearance change, and find near-equivalent alternate sizes.
| Measurement | Original | New | Change |
|---|
| Indicated | Actual | Difference |
|---|
Suggestions stay close to the original tire diameter. Final fitment still depends on your vehicle, wheel width, offset and load requirements.
A tire size comparison calculator helps you compare an original tire with a replacement size before you change wheels or tires. Instead of looking only at the numbers printed on the sidewall, this tool calculates overall diameter, section width, sidewall height, circumference, revolutions per mile, approximate clearance change and speedometer error.
Use the calculator above when you are considering plus-sizing, downsizing, a wider tire, a different wheel diameter, or an alternate replacement size. It gives you a quick mathematical comparison so you can identify combinations that deserve a closer vehicle-specific fitment check.
A size such as 225/50R17 contains three key measurements. The first number is the nominal section width in millimeters. The second number is the aspect ratio, meaning the sidewall height is that percentage of the tire width. The final number is the wheel diameter in inches.
The “R” identifies radial construction. Load index and speed rating, which normally follow the size, are also important for final tire selection even though they are not part of the dimensional calculation.
A common rule of thumb is to keep replacement tire overall diameter close to the original size. A difference within roughly ±3% is often used as a first screening range because larger changes can have a more noticeable effect on speedometer readings, gearing, clearance and vehicle systems.
That does not guarantee a tire will fit. Width, wheel offset, suspension travel, brake clearance, load capacity and manufacturer recommendations still matter.
Changing overall tire diameter changes the distance traveled per wheel revolution. If the new tire is taller than the original, the vehicle travels farther per revolution, so your actual road speed can be higher than the speedometer indicates. If the replacement tire is shorter, actual speed can be lower than the indicated speed.
The basic relationship is: actual speed = indicated speed × (new tire diameter ÷ original tire diameter). The speedometer table in this calculator applies that relationship automatically at several common road speeds.
Plus-sizing means increasing wheel diameter while selecting a lower-profile tire that keeps overall tire diameter close to the factory setup. A properly chosen plus-size can preserve speedometer accuracy while changing steering response, appearance and sidewall height. A larger wheel does not automatically require a taller overall tire.
The clearance figure in this tool is based on half of the overall diameter change. For example, if the replacement tire is 1 inch taller overall, its radius is approximately 0.5 inch larger, reducing vertical wheel-well clearance by about 0.5 inch. This calculation does not measure side clearance to struts, control arms, liners or fenders.
Two tires carrying the same nominal size can have slightly different measured widths and diameters. Tire model, tread design, approved wheel-width range, inflation pressure and manufacturer specifications can all affect the final mounted dimensions.
Use this calculator for comparison and planning, then confirm the exact specification sheet for the tire you intend to install.
Keeping overall diameter close to the original size is generally best. Around ±3% is a common screening guideline, but vehicle-specific fitment, load rating, wheel width and clearance should always be checked.
A taller overall tire usually makes actual road speed higher than the indicated speed because each tire revolution covers more distance. The calculator shows the estimated error automatically.
Sometimes, but only if the tire manufacturer approves that tire size for your wheel width and there is adequate inner and outer clearance. Tire width alone is not enough to confirm fitment.
Not necessarily. Plus-sizing uses a larger wheel with a lower-profile tire to keep overall tire diameter close to stock.
Revolutions per mile estimate how many times the tire rotates while traveling one mile. A taller tire typically makes fewer revolutions per mile than a shorter tire.
No. Diameter difference is only one part of fitment. Width, wheel offset, suspension geometry, brake clearance, load rating and available space must also be considered.
If overall tire diameter changes enough to create a meaningful speed error, recalibration may be appropriate. Some vehicles can be adjusted electronically, while others may require dealer or specialist equipment.
